Change review toggle keybinding to 'v'.

So that the review toggle command can have the same keybinding
in the change list and change view, switch it to 'v'.  'r' and 'd'
are other candidates but both make more sense for the 'review' and
'diff' commands in the change view.

Change-Id: I806fd528cc6fa648450d69181aadbacd67548dda
Merge "Change review toggle keybinding to 'v'."